1. Field of Invention
This invention relates to an improved helicopter in which the main rotor pylon and associated fuselage parts are designed to minimize external aerodynamic drag, ram drag penalties, turbulence, tail shake and exhaust gas reingestion problems in the engine.
2. Description of the Prior Art
In the helicopter art for twin engine designs, it is conventional to discharge engine exhaust laterally on opposite sides of the helicopter, thereby creating drag, and to fabricate the helicopter pylon of an aerodynamic shape so that flow separation would occur therealong during flight to produce additional drag, as in U.S. Pat. No. Des. 239,930 granted May 18, 1976 to H. D. Ulisnik and entitled "Helicopter". Similar lateral engine exhaust discharge and curved pylon surface construction is also shown in U.S. Pat. No. Des. 186,796 granted Dec. 1, 1959 to Gluhareff et al and entitled "Helicopter".
In the usual prior art helicopter construction, engine exhaust gases are discharged at relatively low velocities at an angle to each side of the helicopter, typically on the order of 45.degree., at the rear of the engine compartment pylon. This construction has the disadvantage that ram drag (air mass flow times flight velocity) that always occurs at an air inlet would not be counteracted by exhaust momentum, partly because of the low velocity and partly because of the cosine effect of the exhaust angle. There would thus be a net momentum deficiency, of the airflow through the engines which would add to the aircraft external aerodynamic parasite drag. The second disadvantage of this prior art construction is that the exhaust flow would trigger flow separation over the rear of the pylon behind the exhaust, adding to the external drag. The third disadvantage of this prior art construction is that the flow in the area behind the exhaust would typically by turbulent, leading to possible tail shake difficulties in certain flight regimes. The fourth disadvantage was that the exhaust might tend to recirculate under some flight conditions in and around hover, causing an engine temperature rise with consequent loss of engine power.
The prior art also utilized selectively shaped pylons to abate flow separation but these selective shapes add to manufacturing cost. Such a construction is shown in U.S. Pat. No. 3,008,671 granted Nov. 14, 1961 to Schneiter and entitled "Main Rotor and Pylon Fairing".